1) Congratulations on keeping your sodium intake down!
2) Either drink something other than water, tea or coffee (which have basically the same nutritional value as water), or eat something, too. Unless you're going out of your way to avoid sodium in your diet, just about anything will do.
3) Check with your pharmacist to make sure none of the medications you take affect sodium levels. Many drugs used to control blood pressure, for instance, do.
4) Ask your doctor. Seriously. S/he should know more than random people on the Internet, even on a great forum like this one.
5) Pay attention to how you feel. Cramps, dizziness, unusual fatigue, or anything else that just feels "off" should be taken seriously, especially if you're doing something you don't do often or under unusual (for you) conditions like elevation or temperature.